1. Field of the Invention
The present invention generally relates to the technique for preparing single crystals for use as semiconductor substrates. More particularly, the present invention relates to a method of feeding dopant in the continuously-charged method and a dopant composition used thereby.
2. Description of the Related Art
At present, most of semiconductor substrates used for fabricating integrated circuits are made of single crystal silicon. However, a substantial percentage of the silicon crystals used by the semiconductor industry are prepared by the Czochralski technique, CZ thereafter. In the CZ crystal-growth process, polycrystalline silicon is first loaded into a crucible, which is placed in the chamber of a manufacturing apparatus, as a raw material. A main heater around the crucible is utilized to heat the polycrystalline silicon so as to melt above its melting point. Moreover, a suitably oriented seed crystal is suspended in the crucible by a seed holder, and thereafter is immersed into the melt. It is then slowly pulled up and rotated in a direction the same as or counter to that of the crucible rotation. Progressive single -crystal growing is performed at the solid-liquid interface.
However, dopant, such as boron, phosphorus, or antimony, is added to the melt so that the grown single crystal obtains a desired conductivity type, either N-type or P-type. For example, a known amount of granular dopant is cast into the melt when a single crystal is manufactured by the CZ method and should be doped into a desired concentration. Moreover both granular polysilicon and dopant are added to the melt by the continuously-charged method during the fabrication of silicon crystals.
The continuously-charged method, which replenishes the crucible with raw material based upon the volume of the grown single crystal, can efficiently prepare large-diameter single crystals of silicon. FIG. 3 schematically depicts an apparatus for preparing single crystals by melting rod-shaped polysilicon as the provision of the raw material for the continuously-charged method in a partially sectional view. As shown in the drawing, a single crystal 4 is pulled out from the central portion of a crucible 3. Two heaters 5 are spaced apart over the crucible 3 and the single crystal 4 is placed therebetween. Rod-shaped polysilicon 1, named as polysilicon rods herebelow, are suspended from the upper portion of a chamber 6 and put in those heaters 5, one by one, so as to melt them down. The polysilicon rods are then melted and drop into the melt 7. After one of the polysilicon rods is completely melted, another polysilicon rod starts to melt, thus continually providing the raw material. Reference numerals 8 designate protection casings, each of which encloses the corresponding heater 5, provided with the bottom portions merged in the melt 7 so as to avoid the vibration propagation caused by the dropping as well as to perform gas phases detachment. Furthermore, reference numeral 9 designates a main heater and reference numeral 10 indicates a heat-preservation barrel. Reference numeral 11 is the shaft of the crucible.
When the continuously-charged method is utilized to prepare the single silicon crystals of either P-type or N-type, both the granular dopant and polysilicon are directly cast into the melt. However, those cast articles may float over the surface of the melt because of insolubility. To solve this problem, dopant-containing polysilicon as raw material can be melted and fed to the melt. In other words, the polysilicon rods used in the continuously-charged method have been doped with the same desired doping concentration as the grown crystal. Therefore, by heating the doped polysilicon rods to the melt, single crystals of silicon with the desired and lengthwise uniform doping concentration are achievable. Moreover, a simple and convenient way is utilized to coat a water solution, which contains the oxide of dopant, onto the outer periphery of the polysilicon rods.
However, a portion of the dopant coated onto the polysilicon rods may be vaporized during heating. Accordingly, it is difficult to fabricate single crystals of silicon provided with desired and also uniform doping concentration along the longitudinal length.